The present invention relates to inner closed loop power control. Although not restricted thereto, it is particularly useful in a WCDMA TDD (Wideband Code Division Multiple Access Time Division Duplex) system. It can therefore be usefully applied to the UTRA (Universal Terrestrial Radio Access) TDD downlink (DL) inner loop power control being deployed according to standards developed by the 3GPP (3rd Generation Partnership Project for producing WCDMA standards).
Inner closed loop power control is used in the UTRA for DL, in which UE (User Equipment) measures signal-to-interference ratio (SIR) and according to that measurement sends transmission power control command (TPC) to a base station (BS). The base station may adjust its transmission power according to a TPC command send by the UE with a power control step that is signaled from RNC (Radio Network Control) when it is within the range of operation.
In UTRA TDD users are separated in both code and time domain. Due to this time division mode channel conditions can fluctuate significantly. Unlike WCDMA FDD inner loop power control that has speed of 1500 Hz, the slow power control speed (100 Hz) in UTRA TDD is unable to counter fast channel fading. Downlink power control performance can be enhanced by increasing inner loop power control command rate, but a fast inner loop would ruin the TDD asymmetry and dynamic nature of the system. Although UTRA FDD (Frequency Division Duplex), uses the same kind of closed loop power control as in TDD, it does not require multiple steps because transmission is continuous and power control commands can be sent in every slot thus power control can follow channel conditions even with fixed steps.
According to current 3GPP specifications downlink power control step size is signaled from RNC to the base station, called Node B in the 3G WCDA system. Currently only one step size is signaled from RNC to the base station concurrently. The step size cannot be changed fast enough to react to changes in channel conditions and interference situations.
In TDD the fast fading is presently countered by transmitting excess power to counter the fading and compensate for the slow control loop. This naturally leads to extensive inter-cell interference conditions. Currently, inner closed loop power control uses a power control word for UP and DOWN. These commands are signaled according to 3GPP Release 4 standard with two bits 11 and 00 for providing maximum error distance. This decision is made based on comparison to a SIR threshold.
A problem with this signaling scheme occurs when the rate of the channel change is faster than the power control feedback rate. New signaling schemes could be devised to effect a different power control methodology but there would then be no solution or rules on how old equipment would judge and act when receiving a signaling bit combination that would not be recognized as a valid power control word.